Mixing device for an exhaust system

ABSTRACT

A mixing device for mixing a gas and a liquid in an exhaust system has an injector for injecting the liquid, a shell, an inlet, an outlet and an exhaust gas flow path between the inlet and the outlet. The mixing device defines an axis, and the flow path has a first part, a second part and a third part. The first part extends generally parallel to the axis to direct a flow in a generally forward direction, the first part extending up to a first axial position. The second part extends generally parallel to the axis to direct flow in a generally reverse direction. The third part extends generally parallel to the axis to direct a flow in the generally forward direction. The third part is a volume that allows mixing and extends past the first axial position.

RELATED APPLICATIONS

The application is the U.S. National Phase of PCT/GB2007/000844 filed 9 Mar. 2007, which claimed priority to UK Application 0606116.2 filed 28 Mar. 2006.

BACKGROUND OF THE INVENTION

The present invention relates to a mixing device for exhaust systems and also a method of treating exhaust gases of an internal combustion engine.

To clean up emissions from vehicles such as trucks and cars it is known to use diesel particulate filters (DPF) and selective catalytic reduction (SCR).

With regard to DPFs, in order to burn off the accumulated carbon on the DPF, it is known to inject diesel fuel or other hydrocarbons in front of a diesel oxidizing catalytic (DOC) to create heat by catalytic oxidation. This heat then passes from the DOC to the DPF raising the temperature of the DPF and hence burning off the accumulated carbon.

SCR is used to remove oxides of nitrogen (NOx). In this case urea, or a similar liquid is injected upstream of the SCR catalyst to act as a chemical reductant to remove NOx.

For either system to work reliably and effectively it is necessary that the injected liquids are highly dispersed and are evenly distributed onto the catalyst. However, on a typical installation there is little space available to allow good mixing to occur. It is known to provide devices to create turbulence to assist mixing, however, these devices cause a relatively high back pressure which adversely effects fuel economy and engine durability.

The present invention seeks to overcome or mitigate some or all of these problems.

SUMMARY OF THE INVENTION

Thus, according to the present invention there is provided a mixing device for mixing a gas and a liquid in an exhaust system. The mixing device has an injector for injecting the liquid, a shell, an inlet, an outlet and an exhaust gas flow path between the inlet and the outlet. The mixing device defines an axis, and the flow path has a first part, a second part and a third part. The first part extends generally parallel to the axis to direct a flow in a generally forward direction, the first part extending up to a first axial position. The second part extends generally parallel to the axis to direct flow in a generally reverse direction. The third part extends generally parallel to the axis to direct a flow in the generally forward direction. The third part is a volume that allows mixing and extends past the first axial position.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 shows a cross section of an exhaust system including a mixing device according to the present invention.

FIG. 2 shows a cross section of another example of a mixing device according to the present invention.

FIG. 3 shows a cross section of another example of a mixing device according to the present invention.

FIG. 4 shows a cross section of another example of a mixing device according to the present invention.

FIG. 5 shows an end view of one portion of FIG. 4.

FIG. 6 shows an end view of another portion of FIG. 4.

FIG. 7 shows an enlarged view of FIG. 4.

FIG. 8 shows another embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 9 shows another embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 10 shows another embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 11 shows a further embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 12A shows a further embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 12B shows a further embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 12C shows a further embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 12D shows a further embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 12E shows a further embodiment of an exhaust system including mixing devices according to the present invention.

FIG. 12F shows a further embodiment of an exhaust system including mixing devices according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows part of an exhaust system 10 including a mixing device 12 and a catalyst 14. The mixing device 12 includes an inlet 16 and an outlet 18. Between the inlet 16 and the outlet 18 there is defined a gas flow path F1, F2, F3. The mixing device 12 also includes an injector 20. The exhaust system 10 is connected to an exhaust manifold (not shown) of an internal combustion engine. When the engine is running, exhaust gases pass down the exhaust system 10 generally from left to right as shown by the exhaust gas arrows EG. When required, a reagent is injected into the exhaust gas flow by injector 20. The injected reagent and exhaust gas then pass through the mixing device 12 and on to the catalyst 14.

Where the catalyst 14 is a DOC, the reagent injected by injector 20 is diesel fuel or another type of hydrocarbon fuel. Where the catalyst 14 is an SCR the reagent injected by injector 20 is urea or an equivalent reagent.

In summary, the mixing device 12 is designed such that reverse flow of the exhaust gas and reagent occurs as they pass through the mixing device 12. Thus, it can be seen that the flow path F1 is generally left to right, the flow path F2 is generally right to left, and the flow path F3 is generally left to right. In this case, there are two general reversals of gas flow, though in further embodiments the mixing device could be designed to have a single reversal of gas flow or three reversals of gas flow or four reversals of gas flow or more than four reversals of gas flow.

The gas flow reversal creates a homogeneous distribution of liquid within the exhaust gas which then passes onto the catalyst 14 with minimum loss of space and minimum back pressure.

In more detail, the mixing device 12 includes a shell 22 having an outer portion 24 made from sheet steel and an inner portion 26 made from a thermal insulation material.

A central tube 30 is positioned within the shell 22 and a sleeve 50 is positioned between the central tube 30 and the shell 22. The left hand (when viewing FIG. 1) end 31 of tube 30 defines the inlet 16. The right hand (when viewing FIG. 1) end 32 of tube 30 is blanked off by blanking plate 33. Between ends 31 and 32 the tube 30 has a perforated region 34. In this case the perforated region comprises over 200 holes 35.

The sleeve 50 includes a central cylindrical portion 51, a frustoconical end portion 52, and a frustoconical end portion 53.

The frustoconical end portion 53 connects the cylindrical portion 51 with the right hand end 32 of central tube 30. There are no perforations in either the cylindrical portion 51 or the frustoconical end portion 52, and as such the frustoconical end portion 52 acts to blank off the right hand end of sleeve 50.

However, the frustoconical end portion 53 defines a perforated region 54 of the sleeve 50. The perforated region 54 includes holes 55. In one example, there are over 200 holes 55. The right hand end 32 of the central tube 30 and the right hand end of the sleeve 50 are both supported by a baffle 60. Baffle 60 includes holes to allow the passage of the exhaust gases, these holes are similar to those shown on baffle 360 on FIG. 5.

A support member 62 includes an outer frustoconical region 63 and an inner frustoconical region 64. There are no perforations in the outer frustoconical region 63 and there are no perforations in the inner frustoconical region 64. The left hand end of the perforated region 54 of the sleeve 50 is connected to the right hand end of the outer frustoconical region 63, and is therefore supported by the outer frustoconical region 63. The left hand end 31 of the central tube 30 is connected to and supported by the right hand end of the inner frustoconical region 64. The inner frustoconical region is, in turn, supported at its left hand end by the outer frustoconical region. The outer frustoconical region 63 includes a portion 65 which supports the injector 20.

Consideration of the perforated region 34 of the central tube 30 and the perforated region 54 of the sleeve 50 show that they do not axially overlap, i.e. there is a gap G between the axial position of the left most hole of the perforated region 34 and the right most hole of the perforated region 54.

It will be appreciated that the exhaust gas and reagent must initially enter the central tube 30 via the inlet 16. At this point, all the exhaust gas is traveling from left to right when viewing FIG. 1. When the exhaust gas has reached the perforated region 34, the exhaust gas flow direction will turn and the exhaust gas will flow radially outwardly through the holes 35 into an inner annular region 27 defined between the central tube 30 and sleeve 50. When in this inner annular region 27 exhaust gases will be forced to move from right to left towards the perforated region 54. Upon reaching the perforated region 54 the exhaust gas will again turn and flow generally radially outwardly through the holes 55 into an outer annular region 28 defined between cylindrical portion 51 and the shell 22. When in this outer annular region 28, the exhaust gases will be forced to move from left to right.

It will therefore be appreciated that the flow path includes the first general reversal of the direction of exhaust flow which general reversal will occur as the exhaust gas passes through holes 35. The flow path also includes a second general reversal of the direction of exhaust gas flow, which will occur typically as the exhaust gas passes through the holes 55.

It will also be appreciated that since the central tube 30, the cylindrical portion 51 and the shell 22 are all cylindrical, and are all concentric, the exhaust gas flow path is substantially symmetrical about a center line CL of the mixing device 12, and this is in spite of the fact that the injector 20 is positioned asymmetrically relative to the center line CL.

It will also be appreciated that there is a space between the catalyst 14 and the blanking plate 33 which defines the outlet 18. Note that final mixing occurs in flow path F3 and in outlet 18 prior to the gas entering the catalyst 14.

Note that flow path F2 generally surrounds flow path F1 and that flow path F3 generally surrounds flow path F2.

FIG. 2 shows an exhaust system 110 in which components that fulfill substantially the same function as those of exhaust system 10 are labeled 100 greater.

In exhaust system 110, the injector 120 injects the reagent at the center line CL of the mixing device 112 whereas the injector 20 (FIG. 1) injects the reagent from an edge of the mixing device 12. It will be appreciated that the flow path F1, F2, F3 shown in FIG. 2 is identical to the flow path F1, F2, F3 shown in FIG. 1.

FIG. 3 shows an exhaust system 210 in which components that fulfils substantially the same function as those of exhaust system 10 are labeled 200 greater. In summary the mixing device 212 is a “mirror image” version of mixing device 12. Thus, the exhaust gas flow is generally from left to right as shown by arrows EG. However, the exhaust gas enters the mixing device at inlet 216 and leaves the mixing device 212 at outlet 218. It will be appreciated that flow F1 is from left to right along outer annular region 228. The flow then reverses by flowing radially inwardly through holes 255 into inner annular region 227 at which point the gas flow is from right to left. The flow then reverses again by flowing radially inwardly through holes 235 and then flows from left to right along the center of central tube 230 and out of the mixing device. The injector is positioned upstream of the mixing device 212, and positions X, Y and Z show examples of where the injector might be positioned.

Note that because the mixing device 212 is a “mirror image” version of mixing device 12, the flow path F1 generally surrounds the flow path F2 and the flow path F2 generally surrounds the flow path F3.

FIGS. 4 to 7 show an exhaust system 310 in which components that fulfill substantially the same function as those of exhaust system 10 are labeled 300 greater.

In this case the injector is not shown, but will be positioned upstream of inlet 316. Holes 335 and 355 are only shown schematically (as crosses).

FIG. 5 shows an end view of the baffle 360 which includes several holes 361 which allow the exhaust gases to pass through the baffle 360 and onto the catalyst 314. A central region 366 of baffle 360 acts to blank off the end 332 of central tube 330. The cross section area of the central tube 330 is A1 (see FIG. 6). The cross section area of the inner annular region 327 is A4 and the cross section area of the outer annular region 328 is A5.

The open area (i.e. the gas flow area) of the holes 335 is A2 and the open area (i.e. gas flow area) of the holes 355 is A3.

Preferably A2 approximately equals A3.

Preferably A2 is greater than or equal to 1.5 times A1.

Preferably A2 is greater than or equal to 1.5 times A4.

Preferably A2 is greater than or equal to 1.5 times A5.

Preferably A3 is greater than or equal to 1.5 times A1.

Preferably A3 is greater than or equal to 1.5 times A4.

Preferably A3 is greater than or equal to 1.5 times A5.

Preferably A4 is approximately equal to A1 or is greater than A1.

Preferably A5 is approximately equal to A1 or greater than A1.

FIG. 9 shows an exhaust system 410 in which components that fulfill substantially the same function as those shown in exhaust system 10 are labeled 400 greater. In this case the injector 420 is positioned in a pipe 470, the diameter of the pipe 470 is smaller than the diameter of the shell 422. A further pipe 471 connects the mixing device 412 with the catalyst 414. The diameter of the pipe 471 is smaller than the diameter of shell 422. The diameter of pipe 471 is also smaller than the casing 472 of the catalyst 414. Pipe 471 can vary in length, and can include one or more bends, depending upon the particular installation.

FIG. 8 shows an exhaust system 510 in which components that fulfill substantially the same function as those shown in 410 are labeled 100 greater. In this case exhaust system 510 is a modified version of exhaust system 410. It can be seen that the mixing device 512 and the catalyst 514 have been integrated into a single unit. Thus, the exhaust system 510 does not include a pipe that would be the equivalent of pipe 471 of exhaust system 410. The integrated exhaust system 510 is more compact than the exhaust system 410, and also includes fewer components (as it does not include pipe 471).

Thus, the exhaust system 510 is integrated because the outlet 418 from the mixing device 512 passes directly to the inlet 573 to the catalyst 514. In other words, the diameter of the shell 522 of the mixing device 512 is substantially the same as the diameter of the casing 572 of the catalyst 514, i.e. when the exhaust gases pass from the mixing device 512 to the catalyst 514, there is no significant reduction in cross section area of flow path.

FIG. 10 shows an exhaust system 610 incorporating two mixing devices 612A, 612B according to the present invention. The exhaust system 610 also incorporates four catalysts 614A-D and a DPF 675.

Thus, catalyst 614A is a DOC, catalyst 614B is an SCR, catalyst 614C is DOC, and catalyst 614D is DOC. The DPF 675 is provided between catalyst 614C and 614D. Injector 620A is a urea injector and injector 620B is a diesel fuel injector. As the exhaust gases pass through the exhaust system 610, they are treated as follows:

-   -   (a) the DOC catalyst 614A oxidizes NO to NO2.     -   (b) Urea is injected at injector 620A and mixed with the exhaust         gas in mixing device 612A.     -   (c) The SCR catalyst 614B then removes NOx.     -   (d) The diesel injector 620B injects diesel into the gas stream         and the mixing device 612B mixes the exhaust gas and the diesel.     -   (e) The exhaust gas/diesel mixture pass into the DOC 614C and         the diesel fuel is oxidized thereby generating heat.     -   (f) The heat is passed into the DPF 675, thereby burning off         accumulative carbon.     -   (g) The exhaust gas then passes into the DOC catalyst 614D to         oxidize any remaining hydrocarbons.

It will be appreciated by those skilled in the art the injector 620A and 620B only inject reagent as and when required. Various sensors on the engine and within the exhaust system will determine when injection of a particular reagent is required and this injection is controlled by a control system.

FIGS. 11 to 12F show an exhaust system 710 in which components that fulfill substantially the same function as those of exhaust system 10 are labeled 700 greater.

The mixing device 712 includes a shell 722. A central tube 730 is positioned partly within the shell 722 and a sleeve 750 is positioned between the central tube 730 and the shell 722. The tube 730 defines an inlet 16. The right hand (when viewing FIG. 11) end 732 of tube 730 is blanked off by blanking plate 733. Tube 730 has a perforated region 734 (shown schematically as a cross).

Sleeve 750 is connected to an extension of blanking plate 733 at its right hand end and includes a perforated region 754 at its left hand end (shown schematically as a cross).

Consideration of the perforated region 734 of the central tube 730 and the perforated region 754 of the sleeve 750 show that they do not axially overlap, i.e. there is a gap G′ between the axial position of the left most hole of the perforated region 734 and the right most hole of the perforated region 754.

An injector (not shown) is included in central tube 730 to inject a reagent.

In use, exhaust gas and reagent are mixed in the mixing chamber and the flow is similar to that of exhaust system 10, i.e. the exhaust gas and reagent initially travel from left to right until the perforated region 734 is reached, whereupon the exhaust gas flow production will turn and the exhaust gas will flow radially outward through the holes in the perforated region 734 and into the annular region 727 defined between the central tube 730 and the sleeve 750. When in this inner annular region 727 exhaust gases will be forced to move from right to left towards the perforated region 754. Upon reaching the perforated regions 754 the exhaust gas will again turn and flow generally radially outwardly through the holes in the perforated region 754 into an outer annular region 728 defined between the cylindrical portion 751 and the shell 722. When in this outer annular region 728, the exhaust gases will be forced to move from left to right, and will ultimately pass the blanking plate 733.

FIG. 11 shows various axial positions A1″, B1′, C1′, D1′, E1′ and F1′ of the mixing chambers at the appropriate positions. For the purposes of explanation, we can consider a portion (or slug) of exhaust gas S traveling through the mixing chamber. At position A1′, the slug is contained within the central tube 730 which has cross section area A1′. As the slug of gas moves to the B1′ position, it expands, since it is no longer constrained by the central tube 730, and is only constrained by the sleeve 750. As shown in FIG. 12B, the slug occupies a cross section area of the mixing chamber equivalent to A1′ plus A4′.

Once the slug of gas has reached position C1′, whilst it is still constrained within sleeve 750, it is constrained on its inner diameter by tube 730. The gas is therefore flowing through an area A4′ which is necessarily smaller than the volume at position B1′.

Once the slug of gas reaches the position D1′, it is no longer radially constrained by sleeve 750 and can therefore expand radially outwardly to occupy the volume as shown in FIG. 12D, i.e. the volume cross section area equivalent to A4′ plus A5′. Continued flow of the exhaust gas to position E1′ again causes a narrowing of the cross section of flow area to A5′. Finally, as the slug of exhaust gas passes blanking plate 733, it can expand into the outlet (position F1′) which has a cross section flow area equivalent to A1′ plus A4′ plus A5′.

In summary, the slug S, starting at position A1′, will expand when it reaches position E1′, and then will contract when it reaches position C1′, and then will expand when it reaches position D1′, and then will contract when it reaches position E1′ and then will expand when it reaches position F1′.

The mixer therefore causes the gas to expand then contract then expand then contract then expand, and this process of repeated expansion and contraction attenuates the exhaust gas noise.

Consideration of FIG. 11 shows that the diameter of tube 730 is L1. The radial distance between tube 730 and sleeve 750 is L2. The radial distance between sleeve 750 and shell 722 is L3.

The length over which tube 730 is perforated is M1. The length over which sleeve 750 is perforated is M2. It will be noted that M1 is larger than L1 and is also larger than L2.

Furthermore, the open area of the perforated region 734 is larger than A1′ and is also larger than A4′. The open area of perforated region 754 is larger than area A4′ and is also larger than area A5′.

In this manner, the mixing chamber 712 can be arranged to expand, then contract, then expand, then contract, then expand exhaust gas as it passes through the mixing chamber 712. A similar process of expansion and contraction and expansion and contraction and expansion occurs as exhaust gas pass through the other embodiments shown in the accompanying figures.

Turning to FIG. 1, there are over 200 holes 35, and there are also over 200 holes 55. Depending upon the particular circumstances, there may be more or less holes 35 and there may be more or less holes 55. Typically there will be more than 100 holes 35 to provide for perforated region 34, and typically there will be over 100 holes 55 to provide for perforated region 54. However, in further embodiments the perforated region 34 of central tube 30 could be completely removed, thereby creating a simple gap for the gases to pass through. Similarly, the perforated region 54 of the sleeve 50 could be completely removed, thereby creating a simple gap for the exhaust gases to pass through. Clearly, such modifications could be applied to any of the embodiments shown i.e. any of the set of perforations could be removed to create a simple gap for the gases to pass through.

Whilst the embodiments shown provide a substantially symmetrical flow path, in further embodiments this need not be the case.

As shown in FIG. 1, the perforated region 34 does not axially overlap with the perforated region 54. However, in further embodiments these perforated regions could axially overlap whilst nevertheless ensuring that there is still a general reversal of the exhaust gas flow within the mixing device.

Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. 

1. A mixing device for mixing a gas and a liquid in an exhaust system, said mixing device comprising: an injector for injecting a liquid, wherein said mixing device has a shell, an inlet, an outlet, and an exhaust gas flow path between said inlet and said outlet; and wherein the mixing device defines an axis with said exhaust gas flow path having a first part, a second part, and a third part, said first part extending generally parallel to said axis to direct a flow in a generally forward direction, said first part extending up to a first axial position, said second part extending generally parallel to said axis to direct flow in a generally reverse direction, and said third part extending generally parallel to said axis to direct a flow in said generally forward direction wherein said third part is a volume that allows mixing and extends past said first axial position.
 2. The mixing device as defined in claim 1 in which said exhaust gas flow path is substantially symmetrical about a center line of said mixing device.
 3. The mixing device as defined in claim 1 in which said inlet is defined by a central tube at least partially positioned within said shell and the outlet is defined at least partially by said shell.
 4. The mixing device as defined in claim 3 in which said central tube includes a blanked off tube end.
 5. The mixing device as defined in claim 4 in which said central tube includes tube perforations proximate said blanked off tube end.
 6. The mixing device as defined in claim 5 including a sleeve positioned within said shell and surrounding said tube perforations and being spaced from said tube perforations, said sleeve having a blanked off sleeve end proximate said blanked off tube end.
 7. The mixing device as defined in claim 6 in which said sleeve includes sleeve perforations remote from said blanked off sleeve end.
 8. The mixing device as defined in claim 6 in which said third part is in an annulus and said sleeve partially defines said annulus.
 9. The mixing device as defined in claim 4 in which said blanked off tube end is supported by a baffle.
 10. The mixing device as defined in claim 6 in which said blanked off sleeve end is supported by a baffle.
 11. The mixing device as defined in claim 9 in which a portion of said baffle acts to blank off at least one of said blanked off tube end and/or said blanked off sleeve end.
 12. The mixing device as defined in claim 3 in which said central tube is cylindrical.
 13. The mixing device as defined in claim 6 in which said sleeve is cylindrical.
 14. The mixing device as defined in claim 7 in which an open area of the tube perforations is approximately equal to an open area of the sleeve perforations.
 15. The mixing device as defined in claim 7 in which at least one of an open area of said tube perforations and an open area of said sleeve perforations is greater than or equal to 1.5 times a cross section area of said inlet.
 16. The mixing device as defined in claim 1 in which said cross section area of said third part is approximately equal to or greater than said cross section area of said first part, and/or said cross section area of said third part is approximately equal to said cross section area of said second part, and/or said cross section area of said second part is approximately equal to or greater than said cross sectional area of said first part.
 17. The mixing device as defined in claim 7 in which said sleeve perforations do not axially overlap with said tube perforations.
 18. (canceled)
 19. The mixing device as defined in claim 1 in which an outlet is defined by a central tube positioned within said shell and an inlet is defined by an annulus, said annulus being partially defined by said shell.
 20. The mixing device as defined in claim 1, in which the injector is positioned upstream of the mixing device, in particular to inject a reagent at the inlet of the mixing device.
 21. An exhaust system comprising: a mixing device including an injector for injecting a liquid, wherein said mixing device has a shell, an inlet, and an outlet, and an exhaust gas flow path between said inlet and said outlet and wherein said injector is positioned upstream of said mixing device to inject a reagent at said inlet; wherein said mixing device defines an axis with said flow path having a first part, a second part, and a third part, said first part extending generally parallel to said axis to direct a flow in a generally forward direction, said first part extending up to a first axial position, said second part extending generally parallel to said axis to direct flow in a generally reverse direction, and said third part extending generally parallel to said axis to direct a flow in said generally forward direction wherein, said third part is a volume that allows mixing and extends past said first axial position; and an integrated catalyst. 22.-143. (canceled) 